Quick Notes to Help with Debugging

Reducing

One of the most important steps is reducing the code sample to a minimal reproduction. For example, one thing I'm debugging right now was reported as:

a, b, c, d, e, f, g, h, i, j = 1, *[p1, p2, p3], *[p1, p2, p3], *[p4, p5, p6]

This original sample has 10 items on the left-hand-side (LHS) and 1 + 3 groups of 3 (calls) on the RHS + 3 arrays + 3 splats. That's a lot.

It's already been reported (perhaps incorrectly) that this has to do with multiple splats on the RHS, so let's focus on that. At a minimum the code can be reduced to 2 splats on the RHS and some experimentation shows that it needs a non-splat item to fail:

_, _, _ = 1, *[2], *[3]

and some intuition further removed the arrays:

_, _, _ = 1, *2, *3

the difference is huge and will make a ton of difference when debugging.

Getting something to compare

% rake debug3 F=file.rb

TODO

Comparing against ruby / ripper:

% rake cmp3 F=file.rb

This compiles the parser & lexer and then parses file.rb using both ruby, ripper, and ruby_parser in debug modes. The output is munged to be as uniform as possible and diffable. I'm using emacs' ediff-files3 to compare these files (via rake cmp3) all at once, but regular diff -u tmp/{ruby,rp} will suffice for most tasks.

From there? Good luck. I'm currently trying to backtrack from rule reductions to state change differences. I'd like to figure out a way to go from this sort of diff to a reasonable test that checks state changes but I don't have that set up at this point.